The conventional approach to, or prior art of optical filtering is usually of one of two kinds. The first, spatial optical filtering, is exemplified by instruments such as telescopes, microscopes and cameras. The second kind, spectral optical filtering, is exemplified by sunglasses.
Spatial light filters use lenses to create a virtual image that can be seen by the user, or a real image that is projected on a screen or a photosensitive plate. These instruments are bulky because of the optical limitations of the component lenses: dimensions associated with these instruments must be roughly equal to the sum of the lenses' focal lengths. This problem is partially resolved by the use of prisms, but this solution increases the weight.
Spectral filters use a light modifying layer inserted between the scene and the eye or detector. This results in a spatially uniform attenuation of all the rays reaching the eye or detector. Adaptable spectral filters use phototropic materials that allow, for example, the attenuation to vary with the intensity of the light. These filters are exemplified by photochromic sunglasses. While the attenuation may vary in time with the intensity of the incident light, it is uniform in space for all rays reaching the eye.
Filters have been designed that combine both spatial and spectral filtering. They use a layer of phototropic material located at the focal plane of an optical system. This was proposed in U.S. Pat. No. 3,714,430 by Finvoid et al. and U.S. Pat. No. 3,020,406 by Whitney. Lenses in these filters project an image of the scene on special phototropic material located at the focal plane of the lenses. Light rays originating from bright objects generate dark spots on the phototropic layer and are automatically dimmed by the same spots they generate. Thus, bright rays are self attenuated and dim rays are unaffected. Because of the nonlinear attenuation of the phototropic material, these filter devices permit light detectors to function within their operational dynamic range and prevent bright sources beyond their dynamic range from saturating or damaging the detectors.
These filters use conventional optics and can be bulky. Their field of view is limited. There is a need to extend this technology to provide a very compact solution and widen the field of view. There is also a need to extend this technology to protect human eyes in addition to inanimate detectors.
Very compact and light optical spatial filters have been built using microlenses with the main domain of application being photocopying machines. U.S. Pat. No. 3,658,407 by Ichiro Kitano et al, describes an image transmitter comprised of a bundle of optical fibers made of glass or synthetic resin in which each fiber has an index of refraction that varies parabolically outward from the fiber central axis. Each fiber acts as a focusing lens to transmit part of an image of an object placed near one end. The fiber lenses are produced under the trade name "SELFOC"; the mark is registered in Japan and owned by the Nippon Sheet and Glass Co. Ltd.
This idea is further developed in U.S. Pat. Nos. 4,331,380 and 4,435,039 by James Rees, in which the slant and length of each optical fiber are used to control the device magnification.
As described by prior art, devices using microlens arrays are only capable of performing spatial optical filtering. They do not address the problem of simultaneous spatial and spectral optical filtering.
Unresolved problem areas include, in the domain of combined spectral and spatial filtering, improved car rear view mirrors, car and plane sun visors, windshields, sunglasses, glasses for night driving, laser goggles, nuclear goggles, color shifting goggles (i.e., UV to light), hat sun visors, sun shields, space suit helmets and visors, space station windows, optical instrument protection devices, window shields; in the domain of energy conservation: roofing materials and energy conservation transparent panels.